Calibration-free method of recovering the time-varying absorbance profile in TDLAS by the Fourier reconstructed spectroscopy

Traditional scan-direct absorption spectroscopy (scan-DAS) is susceptible to 1/f noise, limiting its ability to realize high signal-to-noise ratio (SNR) measurements. This study proposes a calibration-free and noise-immune method, named fast scan–Fourier reconstructed spectroscopy (FC-FRS), capable of improving the SNR of the single-shot absorbance spectrum and reconstructing the time-varying absorbance without decreasing the time resolution. Two validation experiments with CO₂ absorbance varying over time were designed using a shock tube device within the millisecond time duration. In the first validation experiment, CO₂ was gradually released during the combustion of hydrocarbons, resulting in the slowly increased CO₂ absorbance. The average spectral SNR of FC-FRS reconstructed absorbance at 2100 K reached 73.51, which increased by 22.83 % compared to the original scan-DAS spectrum. The CO₂ generation profile extracted from the recovered absorbance agreed well with the prediction of kinetic mechanisms. In the second experiment where the CO₂ absorbance was stable behind the reflected shock at 1312 K, the average spectral SNR of the recovered absorbance line behind the reflected shock was 433.81, which is 44 % higher than that of the scan-DAS spectrum. The averaged fitted line strength from the recovered absorbance curve had a relative error of only 1.07 % compared to the HITAN2020 database. Finally, the proposed method was compared with the 2f/1f method of scan-WMS in a static CO measurement experiment, the FC-FRS showed the similar SNR but better stability over time.